1. Field of the Invention
The invention relates to a ferritic spheroidal graphite cast iron and, more particularly, to a ferritic spheroidal graphite cast iron having an excellent heat resistance and oxidation resistance.
2. Description of the Related Art
Materials of exhaust system components, such as an exhaust manifold of an automobile and a turbocharger of a diesel engine, are subjected to service conditions in which high-temperature heating and cooling are repeated. Therefore, these components require oxidation resistance and thermal fatigue resistance. In recent years, with an increase in power and reduction in fuel consumption of an engine, exhaust gas temperature has further increased, and the above request for oxidation resistance and thermal fatigue resistance is further remarkable.
In terms of low cost and easily moldable characteristic, spheroidal graphite cast iron is used as a material that satisfies oxidation resistance and thermal fatigue resistance. However, ferritic spheroidal graphite cast iron decreases its ductility around 400° C. (intermediate temperature embrittlement phenomenon). This phenomenon is peculiar to spheroidal graphite cast iron.
In consideration of the above, Japanese Patent Application Publication No. 10-195587 (JP-A-10-195587) suggests spheroidal graphite cast iron that includes carbon (C), silicon (Si) and manganese (Mn) as principal components, includes at least magnesium (Mg) as a graphite spheroidization component and includes at least one selected from the group consisting of chromium (Cr), molybdenum (Mo), tungsten (W), titanium (Ti), vanadium (V), nickel (Ni) and copper (Cu) as a matrix reinforcing component, and the remaining portion is made of iron (Fe) and unavoidable impurities, and then the graphite cast iron includes 0.03 to 0.20 percent by weight of arsenic (As).
However, the oxidation resistance of ferritic spheroidal graphite cast iron is considerably poorer than that of austenitic cast iron under high-temperature environment around 800° C.
The oxidation resistance of the material described in JP-A-10-195587 is better than the oxidation resistance of ferritic spheroidal graphite cast iron having a high content of Si; however, it is not sufficient when used as the material of the above described parts. This is because a ferrite phase, which is a matrix of ferritic cast iron, is more easily oxidized at 800° C. or above than an austenite phase, which is a matrix of austenitic cast iron. In addition, the oxidation resistance may be improved by increasing the content of Si; however, with an increase in the content of Si, the thermal fatigue characteristic may be impaired.
In consideration of the above, when austenitic cast iron that has an austenite phase and that includes 35 percent by mass of Ni is used as the material of the above parts, addition of a predetermined amount of Ni increases manufacturing cost of cast iron itself.